1. Field of the Invention
This invention relates to wireless electrical devices. More specifically, the invention is a wireless in-plane strain and displacement sensor requiring no electrical connections.
2. Description of the Related Art
Electrical devices typically utilize a plurality of circuit elements wired together to form a circuit. As is well understood in the art, such electrical devices function for a designed purpose when electric current flows through the circuit. If an unwanted break occurs in the circuit, electric current ceases to flow and the circuit must be repaired or replaced to restore device function. Circuit repair or replacement causes downtime, requires manpower, and can be expensive.
In addition, electrical circuits typically use solder to connect circuit elements to one another. The use of solder poses a number of problems. Solder increases the cost of electrical devices and requires the use of venting and air filtration systems during fabrication due to the toxic nature of solder. Further, the high heat required to melt solder can stress or damage circuit boards, and the presence of toxic solder also poses waste issues when old electrical circuits must be disposed of or recycled. For all of these reasons, the typical electrical device has a number of inherent flaws.
One type of electrical device used in monitoring the “health” of structures (e.g., dynamic structures such as aircraft and other vehicles, static structures such as buildings and bridges, etc.) is known as an electrical strain sensor. An electrical strain sensor directly or indirectly relates any mechanical strain to a change in an electrical response. One of the earliest strain gauge designs used a foil of electrically conductive material. When stretched within a material's elastic limits, the foil's resistance increases as the material's longer and narrower shape increases its electrical resistance. When the material is compressed, it becomes shorter and wider thus decreasing the electrical resistance. Strain is directly proportional to the ratio of change in resistance as compared to the resistance of the sensor when it is not deformed. This property is used to make a strain gauge that requires the strain sensor (i.e., the foil) to be directly electrically connected to a resistance measuring circuit such as a Wheatstone bridge.
Other types of electrical strain sensors include capacitive strain sensors, fiber optic strain sensors, and piezoelectric strain sensors. Capacitive strain sensors the displacement between capacitive plates or between neighboring interdigital electrodes. Similar to resistive strain sensors, strain is directly proportional to the ratio of change in capacitance relative to the non-deformed-sensor capacitance. Fiber optics sensors use Bragg gratings that alter the wavelength at which light is reflected and/or transmitted through the fiber. During strain, the grating separation distance changes thus changing the Bragg wavelength (reflected wavelength). The change in wavelength is correlated to strain. The direct optical change can be related to an electrical signal using optoelectronics. A piezoelectric strain sensor uses the changing resistivity of a semiconductor caused by applied strain. All of the above sensors require being part of closed electrical circuits for power and interrogation. Further, because solder and printed circuit boards are typically used to make closed circuits for the sensors discussed above, any reliability, hazardous material, and waste issues associated with solder directly affect them.